Molecular sieves are classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework type zeolites and other crystalline microporous molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the Atlas of Zeolite Framework Types, 6th edition, Elsevier, London, England (2007).
One known molecular sieve for which a structure has been established is the material designated as MTT, which is a molecular sieve having pores defined by parallel channels having cross-sectional dimensions of about 4.5 Angstrom by about 5.2 Angstrom. Examples of MTT framework type molecular sieves include ZSM-23, SSZ-32, EU-13, ISI-4 and KZ-1. MTT framework type materials are of significant commercial interest because of their activity as catalysts in dewaxing of paraffinic hydrocarbons.
ZSM-23, the first molecular sieve isolated with an MTT framework type, and its preparation in the presence of a pyrrolidine directing agent are taught by U.S. Pat. No. 4,076,842, the entire disclosure of which is incorporated herein by reference. According to the '842 patent, ZSM-23 has a composition, in the anhydrous state, expressed in terms of mole ratios of oxides, as follows:(0.58-3.4)M2/nO:Al2O3:(40-250)SiO2 wherein M is at least one cation having a valence n.
U.S. Pat. No. 4,490,342 discloses that ZSM-23 can be synthesized from a reaction mixture containing an organic cation of the formula (CH3)3N+—R1—N+(CH3)3 wherein R1 is a saturated or unsaturated straight chain hydrocarbon group having seven carbon atoms (commonly referred to as Diquat-7). In the Examples, the ZSM-23 products have a silica to alumina molar ratio between 41 and 420.
U.S. Pat. No. 5,332,566 discloses that ZSM-23 can be synthesized from a reaction mixture having a silica to alumina molar ratio between 30 and 300 and containing an organic cation of the formula (CH3)3N+CH2CH2CH2N+(CH3)2CH2CH2CH2N+(CH3)3 (commonly referred to as Triquat-7). Product aluminum concentrations are not reported and all the Examples operate at reaction mixture SiO2/Al2O3 ratios of 45 or above.
In addition, U.S. Pat. No. 5,405,596 reports the synthesis of ZSM-23 in the presence of Diquat-12, (CH3)3N+(CH2)12N+(CH3)3, but using reaction mixtures having silica to alumina molar ratios in excess of 60. Again product aluminum concentrations are not reported.
As noted above, MTT framework type materials, such as ZSM-23, are useful as dewaxing catalysts and, for such applications, it is found that activity and/or selectivity are generally improved when the molecular sieve product has a low silica to alumina molar ratio and a small crystal size. Unfortunately, however, aluminum can be a crystallization inhibitor with certain zeolites, including MTT framework type materials, so that with most existing synthesis routes it is difficult to produce low silica to alumina molar ratio, that is, less than 30:1, zeolites without significant production of impurity phases, particularly FER framework type materials.
U.S. Pat. Nos. 5,053,373 and 5,252,527 disclose that SSZ-32, which is purported to be a high aluminum content zeolite of the MTT framework type, can be synthesized using N-lower alkyl-N′-isopropyl-imidazolium cations as a directing agent. SSZ-32 is said to have a silica to alumina molar ratio of 20 to less than 40 but the lowest silica to alumina molar silica to alumina molar ratio reported in the Examples is 32.
U.S. Pat. No. 6,099,820 discloses a method of synthesizing MTT framework type zeolites in the absence of organic directing agent and using a reaction mixture having a silica to alumina molar ratio of 20 to 200 and a water to silica molar ratio of less than 8. The zeolite product is said to have a silica to alumina molar ratio greater than 12, but product aluminum concentrations are not reported in the Examples.
U.S. Pat. No. 6,692,723 discloses a zeolite of the MTT framework type comprising crystals with a size of less than 5 μm, at least a portion of which are in the form of aggregates having a granulometry such that the dimension Dv, 90 is in the range 40 nm to 100 μm. The zeolite can be produced using alkylated polymethylene α-ω diammonium compounds as the directing agent and the synthesis Examples produce materials having a Si/Al ratio in the range 18.6 to 18.9 (silica to alumina molar ratio of 37.2 to 37.8).
U.S. Pat. No. 7,157,075 discloses that MTT framework type zeolites can be synthesized using as a directing agent at least one nitrogen-containing organic compound selected from the group consisting of N,N,N′,N′-tetramethyl-propane-1,3-diamine, N-isopropyl-isoamylamine, N,N-dimethyl-N′-isopropyl-ethanediamine, N-isopropyl-propane-1,3-amine, N,N-dimethyl-N′-ethyl-ethanediamine, N,N,N,N′N′-pentamethyl-propane-1,3-diammonium cations, N,N′-diisopropyl-pentane-1,5-diamine, N,N′-diisobutyl-propane-1,3-diamine, N,N,′-di-tertbutyl-pentane-1,5-diamine, N,N′-diisobutyl-heptane-1,7-diamine, N,N,N′,N′-tetramethyl-N′-4-methylpentyl-propane-1,3-diammonium cations, trimethyl-pentyl ammonium cations, ethyl-(3-methyl-butyl)-amine, (3-dimethylamino-propyl)-dimethyl-pentyl ammonium cations, 2-aminoheptane, 3,3′-iminobis(N,N-dimethylpropylamine, and certain polymeric amines. The lowest silica to alumina molar ratio reported for the zeolites of the Examples is 33.
U.S. Pat. No. 7,390,763 discloses a small crystal SSZ-32 zeolite, designated as SSZ-32X, which is reported to have a mole ratio of silicon oxide to aluminum oxide greater than about 20:1 to less than 40:1, crystallites having small broad lathe-like components in the range of 200-400 A, and an external surface area in the range from 80 to 300 m2/gm. The zeolite is produced using a mixture of N-lower alkyl-N′-isopropyl-imidazolium cations and an alkylamine as the structure directing agent. The only synthesis Example produced a zeolite having a silica to alumina molar ratio of 29 (see Example 1).
According to the present invention, it has now been found that Triquat-7 is effective as a directing agent in the production of pure phase MTT framework type zeolites at higher aluminum concentrations than those previously exemplified in U.S. Pat. No. 5,332,566. Moreover, the resultant product, composed of aggregates of small crystals of MTT zeolite with a low silica to alumina molar ratio (typically less than 45), is found to have improved activity and selectivity as a dewaxing catalyst as compared to a catalyst prepared from MTT zeolite crystals with a high Si/Al2 ratio of >100/1.
Although the foregoing comments have focussed on aluminosilicates having the MTT framework type, it will be understood that molecular sieves having the same framework type can be produced in which part or all of the aluminum is replaced with other trivalent elements and part or all of the silicon is replaced with other tetravalent elements.